The liquid crystal display (LCD) device primarily includes liquid crystal material sandwiched in between two opposite substrates. The proper bias is generally introduced to alter the angle of the liquid crystal molecules, thereby changing the optical character of the incident light. The liquid crystal display may display the predetermined pattern through the control of pixels. The applications of the liquid crystal display are quite widespread within our daily life and these devices occupy everyplace where the information is needed to be displayed. For example, the liquid crystal display is adopted to the desk top personal computers, the notebooks, the PDA display, the displays of the handy mobile communication products, the displays of the digital timepieces, the displays of the office electric products or the family electric products, and even adopted to the indoor and outdoor dynamical commercial advertising boards.
The types for driving the liquid crystal display include passive and active driving types. At present, the liquid crystal displays primarily adopt to the active driving type to drive the liquid crystal due to the consideration of high reaction speed. The active driving type liquid crystal display generally employs the thin film transistor (TFT) to control the direction of the molecules of the liquid crystal. However, the manufacturing for the thin film transistor is complicated, it generally needs four to five photolithography processes. The extremely complex half-tone lithography processes or other half-exposure-development is introduced in order to reduce the number of mask for the photolithography processes. However, it comes along with the low yield issue.
An active driving type LCD employs the thin film diode as a driving element. The thin film diode has many advantages such as simple structure, simple process and high yield. The prior arts are incorporated herein for reference, U.S. Pat. Nos. 5,926,236, 5,879,960, 6,040,201, 6,271,050, 6,734,460, and JP Patent Publication No. 08-220563 (1996) and 08-320495 (1996).
FIG. 1A illustrates a part of a top view according to prior art, which uses the thin film diode as a driving element in a pixel of a liquid crystal display. The prior art includes a scanning line 12 and a transparent conductive layer 16 used as a pixel electrode. The overlapping area of the transparent conductive layer 16 and the scanning line 12 is a metal-insulator-metal (MIM) structure. FIG. 1B illustrates a cross-section view taken along the line AA′ in FIG. 1A. The scanning line 12 as the bottom metal layer of the metal-insulator-metal structure is formed on a substrate 10, and an insulating layer 14 is overlapped on the scanning line 12. A defect as indicated by the label 18 in FIG. 1B will be generated during the formation of the transparent conductive layer 16 due to the slope structure of layers 12 and 14 used as the bottom metal layer of metal-insulator-metal structure. The breakdown occurs at the defect 18, thereby causing the “bad point” of the pixel.
A solution is disclosed by the U.S. Pat. No. 5,926,236 to avoid the slope structure and increase the aperture ratio. However, the signal processes of the terminal part are not mentioned. Besides, the current-voltage curve is not symmetric due to the metal materials of the top and the bottom layers are different.
To solve the current-voltage curve that is not symmetric caused by the different metal materials consisting of the top and bottom layers, a method is disclosed to introduce the back-to-back thin film diode to obtain the symmetric current-voltage curve, that is to say, uses two metal-insulator-metal structures. As FIG. 2 shows, a pixel electrode 26 is configured between the first scanning line 22 and the second scanning line 24, and is electrically coupled to the first scanning line 22 by the two similar metal-insulator-metal thin film diodes 30, 32, and the pixel electrode 26 electrically coupled to the second scanning line 24 by two other metal-insulator-metal thin film diodes 34, 36. The metal-insulator-metal thin film diode 30 is formed by a top electrode 40 and a bottom electrode 50. The metal-insulator-metal thin film diode 32 is formed by a top electrode 50 and a bottom electro 42. The metal-insulator-metal thin film diode 34 is formed by a top electrode 44 and a bottom electrode 52. The metal-insulator-metal thin film diode 36 is formed by a top electrode 46 and a bottom electrode 52.
However, this method will increase the complexity of the structure and the steps of the manufacturing process, especially to increase the photolithography process. Furthermore, the additional MIM structure will decrease the area for the pixel electrode to lower the aperture ratio.
Another solution expressed in the JP Patent Application Laid-Open Publication No. 08-320495 uses the anode oxidation method to form the top and the bottom layers with the same materials. However, the manufacturing is more complex and the material is limited to the tantalum (Ta) metal that is available for anode oxidation.
In view of above-mentioned insufficient and deficiencies, what is desired is to provide a novel structure for the thin film diode liquid crystal display to solve these problems.